tools: project-browser: reveal newly created and imported resources

[KaindraDjoemena]

After creating or importing a resource, the project browser now
automatically reveals it.

Resource creation (scripts, units, materials, state machines) was
wired up via compile_and_reveal_resource(). Newly created folders
are also auto-expanded in the project browser.

For imports, the Import delegate signature was extended to pass back
the path of the primary output resource:

public delegate void Import(ImportResult result, string? primary_resource_path = null);

Each importer passes back its primary output on success:

• Texture → .texture
• Sound → .sound
• Font → .font
• Sprite → .unit
• Mesh (.mesh) → .unit
• FBX → .unit

on_import_result() then calls compile_and_reveal_resource() so both
creation and import share the same reveal path.

Closes #201

[dbartolini]

Note for future PRs: you could have force-pushed to #771. It's less work for both of us. Thanks!